In vehicle workshops, it is difficult to easily decide when a compressed air compressor incorporated in a vehicle should be exchanged and replaced by a new one. Usually, there are two criteria for exchange. The first is that the compressor shoots out oil into the compressed air. This dirties the air, but does not necessarily mean that the pump capacity is low. Since dirty air can be seen with the naked eye, it is possible to easily and immediately decide whether it is time to change the compressor.
The second criterion is that the compressor is pumping too slowly; that is to say, that the compressor produces too little compressed air per unit of time. This checking of the pump capacity is more complicated and as of yet, there has not been any simple way of gaining a reliable assessment. The checks which have been carried out in workshops have been imprecise and have not been suitable for various types of vehicle. In workshops, the test has been conducted by coupling an external manometer to the compressed air system of the vehicle and then measuring the time it takes for the compressor to raise the pressure to a certain value. This produces only an approximate time value, since it is not possible to adapt the test with regard to sources of error. For example, the test is not adaptable to the fact that different tank volumes ought to give different time values.
Other sources of error are, for example, that the air supply varies if someone climbs into and out of the car during the measurement. The air volume can also be changed by the passage of air to other reservoirs in the vehicle. Attempts have also been made to define “pump-up-time;” i.e., the time it takes when the compressor starts from a rest position until the motor has been run up to a predefined speed and the system has assumed a predefined pressure, but for practical reasons this has not proved successful in the workshops.
Owing to these difficulties in checking the compressor capacity, the compressor is in many cases changed long before its actual working life has expired. On the one hand, this is a waste of resources, and on the other hand, it is unnecessarily expensive to exchange working compressors solely because their capacity cannot be accurately assessed.